We have a 4yo Cocker Spaniel who has had some guarding behaviour when he was a puppy. All fear based apparently. He bit a couple of times (generally guarding food) We have spent a lot of time training him and giving him lots of love with no recurrence for 3 years. He spends time with friends children (generally with a basket muzzle just in case) and is fine. We had a baby girl last summer and he's been as good as gold with her.
As she's got more curious about him we've encouraged gentle and supervised interaction and he's been super placid. No warning signs that either of us have noticed. We've tried hard to give him lots of cuddles too.
This morning she was crawling in the living room and he was by the door in the hall. As she headed towards him he gave a little grumble and I went to shut the kiddie gate to keep her in the living room. She kept moving towards him and he really growled at her I told him off and sent him to his bed (probably the wrong thing to do) and realised she had a dirty nappy. Changed her then gave him lots of attention again.
DH then had her in his arms and the dog was just watching them while I was petting him so I'm guessing it's jealousy but what else can we do? We've worked so hard to make sure he doesn't feel displaced and I thought we'd cracked it as he'd been so placid.
Can you good people calm me down and give me some good practical advice on how to help him through this?
PLEASE don't tell me to re-home him (or her 
) I want to fix this. He isn't dangerous but I am concerned.